Cellular heat shock proteins (HSPs) enhance viral gene expression for multiple viral families, although the in vivo significance is unknown. The mechanistic basis for HSP-mediated increases in viral gene expression has been addressed only for DNA viruses and retroviruses, until our work with the paramyxoviruses measles (MV) and canine distemper virus (CDV). Data from our previous funding interval show that the major inducible 70 kDa HSP (hsp72) interacts with two discrete binding motifs (Box-2 and Box-3) on the C- terminus of the MV nucleocapsid protein (N), where Box-3 mediates stimulation of transcription by hsp72 but not the observed increases in genome replication. Aim one of the current proposal tests the hypothesis that hsp72 binding to Box-2 mediates stimulation of genome replication, and a structure-based approach will be used to understand mechanisms underlying the separable effects of hsp72 on transcription and genome replication. Aim two determines the in vivo significance of these MV-HSP interactions using a mouse model of brain infection. Elevation of hsp72 levels in neurons at the time of MV challenge results in either enhanced neurovirulence or clearance depending upon mouse strain. It is our hypothesis that hsp72-dependent susceptibility to infection reflects H-2 restricted differences in virus specific T cell responses mediating viral clearance, and the degree of viral hsp72-responsiveness mediated by Box-2 and Box-3. By this model, hsp72-dependent increases in viral antigen expression facilitate adaptive immune responses leading to clearance, although such a protective host response backfires if T cell responses are not adequate to contain the burst in viral replication, leading to increased neurovirulence. In either setting, virus of reduced hsp72 responsiveness would be more prone to establish stable persistent infection. Importantly, these viral and host determinants of neurovirulence have potential broad relevance to other neurotropic Mononegavirales with similar mechanisms of regulated gene expression (e.g., rabies and Borna virus). Relevance to Public Health: Fever induces proteins in the brain that can either protect brain from viral infection or make that viral infection worse. Our work will identify the viral and host determinants of these two possible outcomes so that we may effectively treat the virus infected patient. The experimental system uses measles virus, but the work has potential broad relevance to other viruses that infect brain. [unreadable] [unreadable] [unreadable] [unreadable]